Try some flux,60/40 tin lead solder and you should be ok.Strip the insulation off about 10mm,twist and tin.Do the same with both wires,trim to desired length hold next to each other one one way and one the other and solder.Dont forget the heatshrink first.Unless its really oxidised the flux should be your best friend
I would try and stay away from using push fit crimps because if they go manky they will cause resistance.These are better,second one down.Shrink after crimping,I have some and I think they were from RS .

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